And you're compiling proftpd from source, rather than installing some
prebuilt binary package, yes?
> Is there a core dump going on and if so would it help you to have a look
> at that?

Unfortunately there is no core dump file -- proftpd is specifically
designed to not leave core files around, even in the face of segfaults.
(Since proftpd has access to sensitive information, any core dump it might
create could reveal that sensitive information, which is a Bad Thing.)